1. Field of the Invention
The present invention relates to radio frequency (RF) transponders and radio frequency identification (RFID) systems, and more particularly, to a battery-powered RF transponder having a xe2x80x9cwake-upxe2x80x9d circuit that turns on the RF transponder circuitry upon detection of an interrogating signal from an RFID interrogator.
2. Description of Related Art
In the automatic data identification industry, the use of RF transponders (also known as RF tags) has grown in prominence as a way to track data regarding an object on which an RF transponder is affixed. An RF transponder generally includes a semiconductor memory in which information may be stored. An RF interrogator containing a transmitter-receiver unit is used to query an RF transponder that may be at a distance from the interrogator. The RF transponder detects the interrogating signal and transmits a response signal containing encoded data back to the interrogator. RF and RFID systems are used in applications such as inventory management, security access, personnel identification, factory automation, automotive toll debiting, and vehicle identification, to name just a few.
Such RFID systems provide certain advantages over conventional optical indicia recognition systems (e.g., bar code symbols). For example, the RF transponders may have a memory capacity of several kilobytes or more, which is substantially greater than the maximum amount of data that may be contained in a conventional one-dimensional bar code symbol. The RF transponder memory may be re-written with new or additional data, which would not be possible with a printed bar code symbol. Moreover, RF transponders may be readable at a distance without requiring a direct line-of-sight view by the interrogator, unlike bar code symbols that must be within a direct line-of-sight and which may be entirely unreadable if the symbol is obscured or damaged. An additional advantage of RFID systems is that several RF transponders can be read by the interrogator at one time.
RF transponders may either be xe2x80x9cactive,xe2x80x9d in which they include an internal power source (i.e., battery), or xe2x80x9cpassive,xe2x80x9d in which they do not include a battery and derive their energy entirely from the interrogating signal provided by the RF interrogator. The active RF transponders generally have a greater transmitting range than passive transponders, but have the associated disadvantage of greater bulk due to the inclusion of the battery. The operational life of an active RF transponder is dependent upon the capacity of the battery, and it is generally desirable that an RF transponder have as long an operational life as possible (e.g., longer than five years). Even though the circuitry of the RF transponder draws relatively low current, the battery will quickly run down if the circuitry is powered up continuously. To conserve the battery power, the RF transponder may be placed in a low power (or xe2x80x9csleepxe2x80x9d) mode in between operations. When the RF transponder is interrogated, the RF transponder must then be xe2x80x9cwakedxe2x80x9d or restored to an operating mode from the sleep mode.
A drawback of this type of operation is that the interrogating signal must typically be above a certain threshold level (e.g., greater than 100 mv) in order to generate sufficient current to be detected. Another drawback is that the wake-up circuit itself draws current that runs down the battery. Yet another drawback is that the wake-up circuit is not sufficiently selective, and, as a result, it tends to wake the RF transponder whenever any RF signal is detected. In certain applications, plural passive and active RF transponders may be disposed in close proximity to each other, and it would therefore be desirable to selectively wake only specific ones of the RF transponders while allowing other ones of the RF transponders to remain in the sleep mode.
Accordingly, it would be very desirable to provide a wake-up circuit for an active RF transponder that can be operated with very low current, and which can detect an interrogating signal having very low power (e.g., less than 100 mv). In addition, it would be highly desirable to provide a wake-up circuit that can selectively respond to an interrogating signal having a particular pattern so that only a particular RF transponder is waked.
In accordance with the teachings of the present invention, an active RF transponder is provided with a wake-up circuit that wakes the RF transponder from a sleep state upon detection of an RF interrogating signal. The active RF transponder includes a battery, an antenna adapted to receive RF signals from an interrogator, and electronic circuitry providing the various RF transponder functions of sending/receiving signals and storing data.
A first embodiment of the invention includes a wake-up circuit that periodically checks for the presence of an RF signal at the antenna. The wake-up circuit is coupled to the antenna and includes a switch adapted to selectively couple the battery to the electronic circuitry and provide electrical power thereto upon detection of the RF signals. The wake-up circuit further comprises an oscillator providing a clock signal having a low duty cycle that defines intervals during which the antenna is sampled for presence of the RF signals (e.g., approximately 20 ns every 100 xcexcs). The wake-up circuit further comprises a comparator that compares the detected RF signals to a threshold value in accordance with the clock signal and signals the switch to couple the battery to the electronic circuitry upon the detected RF signals meeting the threshold value. After the switch has coupled the battery to the electronic circuitry, the switch becomes uncoupled upon either detection that the RF signals have dropped below a threshold for a predetermined period of time, or upon detection that receipt of a message encoded in the RF signals has completed.
A second embodiment of the RF transponder includes a wake-up circuit as in the first embodiment that is further adapted to detect a code sequence modulated in the RF signals. The code sequence is unique for a class of RF transponder (e.g., battery powered RF transponders), so the wake-up circuit can discriminate between interrogating signals. This embodiment permits the RF transponder to operate within the vicinity of passive RF transponders since only the intended class of RF transponders that correlate with the code sequence modulated in the RF signals will be waked. The battery becomes uncoupled from the electronic circuitry by operation of the switch if the detected code sequence fails to correlate with a predetermined code sequence that is unique to the class of RF. transponders. The wake-up circuit oversamples the RF signals to detect the code sequence modulated therein.
A third embodiment of the RF transponder includes a wake-up circuit that wakes the RF transponder upon detection of an RF signal that contains data within a desired band of frequencies. This embodiment enables the RF transponder to discriminate between RF signals that likely contain valid data and other RF noise. The wake up circuit further comprises an oscillator providing a clock signal having a low duty cycle that defines sampling intervals during which rising or falling transitions of data signals recovered from the RF signal are counted. If the number of transitions that are detected during two successive sampling intervals falls within a desired range (i.e., corresponding to a desired frequency band), the wake-up circuit wakes the RF transponder. After the RF transponder has been waked, the wake-up circuit returns the RF transponder to a sleep state if valid data is not detected within a predetermined period of time.